This invention relates generally to laser gyroscopes and more particularly concerns means for eliminating errors in the gyro output due to "lock-in".
In a laser angular rate gyro, two monochromatic beams of light are caused to travel in opposite directions about a looped path, which extends about the axis of rotation of the gyro. The path is formed by a cavity which is typically polygonal in shape having mirrors disposed at the corners thereof to reflect the beams along the path. As the gyro is rotated, the effective path length for one beam is increased and the effective path length for the other beam is decreased, due to Doppler shifting. A beat frequency is produced in response to heterodyning of the two beams as with a combining prism, the beat frequency signal in turn producing a fringe pattern which is typically detected by a photodiode. The latter produces a sine wave output whose frequency is linearly proportional to the rate of rotation. The beat frequency is given by the expression: ##EQU1## where: .OMEGA. represents the rate of rotation;
A is the geometric area of the laser gyro; PA0 .lambda. is the wavelength of the laser; and PA0 .rho. is the perimeter of the geometric figure described by the lasing path.
Accordingly, the magnitude and sign of the frequency difference, .DELTA.f, are indicative of the rate and direction of rotation of the gyro.
At very low rates of rotation, errors arise due to "lock-in" effects, whereby no frequency difference is observed. Lock-in arises because of imperfections in the lasing cavity, principally in the mirrors, which produce backscatter from one laser beam into the other laser beam. At low rates of rotation where the frequency splitting between the two beams is small, the coupling of the backscatter from one beam into the other beam causes the two beams to oscillate at the same frequency. This results in a deadband, or lock-in region, the lock-in threshold rate being determined by the amount of backscatter. This deadband results in the gyro output not tracking the input. When the gyro input rate of rotation exceeds the threshold of the lock-in rate, the beams separate in frequency and begin to produce output pulses.
Various dither techniques have been employed to eliminate lock-in at low rates of rotation. One such technique is to dither the mirrors located along the cavity path of the gyro as shown in Hutchings U.S. Pat. No. 4,281,930. Hutchings requires that all of the mirrors be dithered at the same frequency but out of phase by an amount equal to 360.degree. divided by the number of mirrors. The phase difference between the dither applied to each of the mirrors must be precisely controlled so as to maintain the cavity path length constant. Because all of the gyro mirrors must be dithered as taught by Hutchings, problems in maintaining the cavity path length constant have arisen since the greater the number of mirrors, the more difficult it is to maintain precise control of the phase difference therebetween.
Another known technique for eliminating lock-in at low rates of rotation is to provide a current dither such as shown in Staats U.S. Pat. No. 3,612,690. The current dither of Staats includes a noise source, the output of which is amplified and limited to provide a rectangular wave randomly variable in frequency and pulse width but constant in amplitude. This rectangular wave and a phase shifted version thereof are applied to a push-pull amplifier, the outputs of which are connected to the anodes of the laser gyro so that the current from each anode is frequency modulated. If the current dither of Staats is employed such that one anode is cut off while the other anode is operating, problems may arise in that there is not enough gain to lase. Even if the two anodes are simultaneously operational, the dither imparted to the current is insufficient to eliminate lock-in at certain rates of rotation.
Although various dither techniques, such as mirror dithers or current dithers, taken alone may prevent lock-in at low rates of rotation near zero, it has been found that nonlinearities occur in the output of the laser gyro when the input rate is a harmonic of the dither frequency, impairing the accuracy of the gyro.